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Superconductor Science and Technology

A new microwave circuit and a new cryoprobe for Josephson voltage standards

M. Schubert, G. Wende, T. May, Ludwig Fritzsch, H.‐G. Meyer

Published Dec 19, 2001• 5 citations
Publisher

Abstract

We present a new type of microwave circuits for Josephson voltage standards. Coplanar strips (CPS) are used instead of microstriplines or coplanar waveguides (CPW) to integrate the Josephson junctions in a microwave transmission line. Compared to microstripline and CPW circuits the CPS design doubles the scale of integration and simplifies the fabrication technology. Well-operating 10 V circuits in CPS design with 17,000 SIS junctions and 1 V SINIS circuits were tested in a new cryoprobe. In this cryoprobe a low loss dielectric waveguide consisting of a teflon strip is used for the microwave transmission in the 70 GHz frequency range. The dielectric waveguide together with a fibreglass-reinforced plastic supporting tube reduces the contribution of the cryoprobe to the liquid helium boil-off rate drastically. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

DOI
10.1088/0953-2048/15/1/320

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IEEE Transactions on Applied Superconductivity•Jun 1, 1995•5 citations
We present a new type of microwave circuits for Josephson voltage standards. Coplanar strips (CPS) are used instead of microstriplines or coplanar waveguides (CPW) to integrate the Josephson junctions in a microwave transmission line. Compared to microstripline and CPW circuits the CPS design doubles the scale of integration and simplifies the fabrication technology. Well-operating 10 V circuits in CPS design with 17,000 SIS junctions and 1 V SINIS circuits were tested in a new cryoprobe. In this cryoprobe a low loss dielectric waveguide consisting of a teflon strip is used for the microwave transmission in the 70 GHz frequency range. The dielectric waveguide together with a fibreglass-reinforced plastic supporting tube reduces the contribution of the cryoprobe to the liquid helium boil-off rate drastically. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.
We present a microwave circuit for Josephson voltage standards. Here, the Josephson junctions are integrated in a microwave transmission line designed as coplanar strips (CPS). The new layout offers the possibility of achieving a higher scale of integration and to considerably simplify the fabrication technology. The characteristic impedance of the CPS is about 50 Ω, and this should be of interest for programmable Josephson voltage standard circuits with SNS or SINIS junctions. To demonstrate the function of the microwave circuit design, conventional 10 V Josephson voltage standard circuits with 17 000 Nb/AlOx/Nb junctions were prepared and tested. Stable Shapiro steps at the 10 V level were generated. Furthermore, arrays of 1400 SINIS junctions in this microwave layout exhibited first-order Shapiro steps.
Improved packages for Josephson Voltage Standard (JVS) circuits have increased operating margins, reliability, and longevity of JVS systems. By using the “flip-chip-on-flex” technique, reliable chip and cryoprobe mounting have been demonstrated. The microwave structures on these packages have been improved such that more power can be delivered to the JVS chip over a wider frequency range: DC to over 30 GHz. Detailed finite-element simulations were performed to optimize the chip-to-flex launches as well as the on-flex transmission lines. It was found that coplanar waveguide transmission line designs had improved insertion and return losses compared to those of the microstrip transmission line designs, in large part due to the large discontinuities associated with through-substrate vias for microstrip ground connections. The improved coplanar-waveguide package/probe yielded insertion loss dominated by the$\sim$0.25 dB/GHz cable loss and VSWR better than 2:1 for the entire 0–30 GHz band. Substantially larger JVS system operating margins were measured using the coplanar-waveguide package; for a 5120 junction array a quantized voltage step greater than 1 mA bias current range is shown for a 10–30 GHz band.
For a more wide-spread use of Josephson voltage standards, cryogen-free operation, by means of an appropriate closed-cycle refrigerator, is highly desirable. In this work we present a low-noise pulse tube cooler (PTC) that is capable to cool the voltage standard circuits to temperatures below 4 K. As a low loss dielectric waveguide with a very small thermal conductivity a Teflon strip was used for the microwave transmission. The matched transition from the WR12 rectangular waveguide to the Teflon strip and vice versa was made using special exponential tapers. Programmable 1 Volt 14 bit SINIS and 10 Volt SIS arrays assembled in the PTC operated well, i.e., demonstrated a similar performance to that measured in liquid helium. The Josephson junction arrays were integrated in coplanar strips instead of the commonly used microstriplines for the microwave transmission lines. The 1 Volt SINIS array contained 8192 Josephson junctions (JJs). It generated an inherently stable Josephson voltage step of 1.19 V at a driving microwave frequency of 70 GHz. The step width was 150 /spl mu/A and the critical current of the junctions was 550 /spl mu/A. Similar results were obtained under liquid helium cooling conditions. In another experiment a 10 Volt voltage standard chip with 19700 SIS JJs was installed. It was possible to generate stable Shapiro steps at the 10 V level.
We have investigated a new type of microwave circuit for the generation of frequency-scaled Josephson voltages useful for the representation of highly accurate reference voltages. The circuit consists of a superconducting microstrip resonator and series arrays of Josephson junctions integrated within its rf current antinodes. The circuit is designed to operate at low frequencies of about 10 GHz to simplify the microwave techniques. At a resonator circuit with four Josephson junctions driven at 10.9 GHz we have obtained stable overlapping Shapiro steps up to 4.6 mV at a step width of about 70 µA. A more advanced double-resonator circuit with altogether 216 junctions allows to generate scaled reference voltages up to 0.16 V at steps of about 25 µA in width and at a microwave frequency of 11.1 GHz.
A new type of cryoelectronic microwave circuits in Nb/Al/sub 2/O/sub 3//Nb technology for the generation of frequency-scaled Josephson voltages is presented. The circuits consist of superconducting microstripline resonators with series arrays of Josephson tunnel junctions located on the rf current antinodes. The circuits are designed to obtain highly accurate Josephson voltages at low rf power levels and low drive frequencies. At present, we have observed Josephson voltages up to 0.66 V in a resonator circuit with 1456 junctions.
We have designed and fabricated a new type of Josephson device to be hybrid integrated with microstripline oscillators. For optimum matching of the Josephson series array to the oscillator the samples make use of a new stripline design. The first circuits were fabricated in a simplified Nb/Al/sub 2/O/sub 3//Nb technology and exhibit excellent dc characteristics and good microwave coupling to the junctions of the array. At 70 GHz constant voltage steps up to 350 mV could be observed. By hybrid integrating different samples with a new 10-GHz oscillator, constant voltage steps were created. For this experiment the Josephson current density had to be low enough to reduce the plasma frequency of the junctions to well below the driving frequency.>